New supramolecular structural motif coupling a ruthenium(II) polyazine light absorber to a rhodium(I) center

Two new complexes, [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 and [(Me2bpy)2Ru(dpp)Rh(I)(COD)](PF6)2(BF4) (bpy = 2,2'-bipyridine, Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, and COD = 1,5-cyclooctadiene), representing a new Ru(II),Rh(I) structural motif, have been prepared and characterized by mass spectrometry, (1)H NMR spectroscopy, electrochemistry, electronic absorption spectroscopy, and emission spectroscopy. These two complexes represent a new type of supramolecular complex with a [(TL)2Ru(dpp)](2+) (TL = terminal ligand) light absorber (LA) coupled to a Rh(I) center and are models for Ru(II),Rh(I) intermediates in the photochemical reduction of water using dpp-bridged Ru(II),Rh(III) photocatalysts. Electrochemical study reveals overlapping reversible Ru(II/III) and irreversible Rh(I/II/III) oxidations and a quasi-reversible dpp(0/-) reduction, demonstrating that the lowest unoccupied molecular orbital (LUMO) is dpp(π*) based. The COD ligand is sterically bulky, displaying steric repulsions between hydrogen atoms on the alkene of COD and dpp about the square planar Rh(I) center. An interesting reactivity occurs in coordinating solvents such as CH3CN, where Rh(I) substitution leads to an equilibrium between the Ru(II),Rh(I) bimetallic and [(TL)2Ru(dpp)](2+) and [Rh(I)(COD)(solvent)2](+) monometallic species. The electronic absorption spectra of both complexes feature transitions at ca. 500 nm attributed to a Ru(dπ) → dpp(π*) metal-to-ligand charge transfer (MLCT) transition that is slightly red-shifted from the Ru synthon upon Rh(I) complexation. The methylation of TL on the Ru impacts the electrochemical and optical properties in a minor but predictable manner. The photophysical studies, by comparison with the model complex [{Ru(bpy)2}2(dpp)](PF6)4 and related Rh(III) complex [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3, reveal the expected absence of a Ru(dπ) → Rh(dσ*) (3)MMCT state (metal-to-metal charge transfer) in the title complexes, which is present in Rh(III) systems. The absence of this (3)MMCT state in Ru(II),Rh(I) complexes results in a longer lifetime and higher emission quantum yield for the Ru(dπ) → dpp(π*) (3)MLCT state than [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3. Both complexes display photocatalytic hydrogen production activity in the presence of water and a sacrificial electron donor, with the [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 possessing a higher catalytic activity than the methyl analogue. Both display low activities, hypothesized to occur due to steric crowding about the Rh(I) site.